make the build system & compilation docs a bit better

CMakeLists.txt

@@ -6,11 +6,19 @@ set(PROJECT ${BOARD}-${PROJECT})
 set(TOP "./tinyusb")
 get_filename_component(TOP "${TOP}" REALPATH)
 
+option(USE_SYSTEMWIDE_PICOSDK "Use the systemwide Pico SDK instead of relying on the one from a deeply nested Git submodule (OFF by default)" OFF)
+
 # Check for -DFAMILY=
 if(FAMILY STREQUAL "rp2040")
   cmake_minimum_required(VERSION 3.12)
-  #set(PICO_SDK_PATH ${TOP}/hw/mcu/raspberrypi/pico-sdk)
-  include(pico_sdk_import.cmake)
+
+  if (USE_SYSTEMWIDE_PICOSDK)
+    include(pico_sdk_import.cmake)
+  else()
+    set(PICO_SDK_PATH ${TOP}/hw/mcu/raspberrypi/pico-sdk)
+    include(${PICO_SDK_PATH}/pico_sdk_init.cmake)
+  endif()
+
   project(${PROJECT})
   pico_sdk_init()
   add_executable(${PROJECT})

README.md

@@ -21,7 +21,7 @@ For BOARD=stm32f072disco, the inexpensive [32F072BDISCOVERY evaluation board](ht
 After initially downloading this project's code, issue the following command to download TinyUSB and CMSIS_5 code:
 
 ```
-git submodule update --init
+git submodule update --init --recursive
 ```
 
 Follow the TinyUSB build instructions [available here](https://github.com/hathach/tinyusb/tree/master/docs), but issue the make command in the base directory of Dapper Mime.
@@ -35,6 +35,11 @@ mkdir cmake-build && cd cmake-build
 cmake -DBOARD=raspberry_pi_pico -DFAMILIY=rp2040 -DCMAKE_BUILD_TYPE=Debug ..
 ```
 
+If you have the Pico SDK installed on your system, and the `PICO_SDK_PATH`
+environment variable is specified properly, you can omit the `--recursive` flag
+in the `git submodule` invocation (to avoid many many git clones), and pass
+the `-DUSE_SYSTEMWIDE_PICOSDK=On` flag to CMake, too.
+
 ## Usage
 
 The pin mapping is as follows: